COMBINING CLASSICAL THERMOBAROMETRY WITH SIMULTANEOUS ACQUISITION OF TRACE ELEMENT AND ISOTOPIC COMPOSITIONS FOR THERMOMETRY AND U-PB GEOCHRONOLOGY BY LASER ABLATION; AN EXAMPLE FROM THE TAUREAU SHEAR ZONE, CENTRAL GRENVILLE PROVINCE, QUEBEC

The Taureau shear zone (TSZ) separates the Mekinac-Taureau domain (MTD) from the overlying Shawinigan domain in the Mauricie area of the Central Grenville Province. On the western flank of the MTD, it was interpreted as an oblique-thrust-sense shear zone, whereas preliminary results conducted on the eastern flank rather suggested a normal displacement. This study investigates the tectono-metamorphic evolution of the shear zone on the eastern flank of the MTD.

Lenses of paragneiss containing the peak metamorphic assemblage quartz-plagioclase-K-feldspar-garnet-biotite-sillimanite were sampled to evaluate the metamorphic contrast between both sides of the TSZ. In contrast to most migmatitic pelitic schists, garnet zoning profiles show weak evidence of diffusional Fe-Mg exchange and retrograde net-transfer reaction with a melt phase during cooling. Metamorphic conditions calculated with TWQ 2.32 yielded 840°C at 1050 MPa in the core of the MTD, 750-780°C at 750-800 MPa close to the TSZ, and 700-750°C at 650-800 MPa in the Shawinigan domain. These results are consistent with higher peak metamorphic conditions in the footwall of the TSZ.

A key sample from the TSZ is an amphibolite containing garnet surrounded by sigmoidal plagioclase+hornblende coronas. Peak conditions of 750-775°C at 1000-1100 MPa were calculated with TWQ v. 1 by combining chemical compositions of high-grossular garnet outer cores with plagioclase and hornblende from the groundmass. In contrast, retrograde conditions of 675-700°C at ca. 700 MPa were calculated by combining chemical compositions of low-grossular garnet rims with plagioclase and hornblende from the coronas. The top-down-to-the-southeast sense of shear indicated by the sigmoidal shape of the coronas implies normal-sense shearing synchronous with, or postdating the development of the coronas.

Laser ablation (LA-ICP-MS) U-Pb zircon geochronology and Ti-in-zircon thermometry conducted on a syn-kinematic pegmatite dyke from the TSZ yielded an age of ca. 1050 Ma and a temperature of ca. 760°C, respectively. This temperature indicates that the dyke intruded during the metamorphic peak. As opposed to previous studies, the present work indicates that the TSZ was active as a normal-sense shear zone ca. 1050 Ma, coeval with the exhumation of 14-18 km (400-500 MPa) of the MTD.